1. Field of the Invention
The present invention relates to an image correlation displacement sensor, and more particularly, to an image correlation displacement sensor that captures speckle fields to measure a displacement of an object.
2. Description of Related Art
Image correlation displacement sensors using a correlation between a plurality of speckle fields (speckle images) are known (see U.S. Pat. No. 6,642,506, U.S. Pat. No. 7,295,324, and U.S. Pat. No. 7,515,280). In general, a coherent light source such as a laser light source is used for each image correlation displacement sensor. A speckle field is produced by illuminating an optically rough surface with the coherent light source. Specifically, a target surface of an object to be measured is illuminated with coherent light, and light scattered from the target surface is detected by a detector such as a CCD camera or a CMOS image sensor. Thus, the speckle field can be captured in an image or images.
First, a speckle field before displacement is captured, and the captured speckle field is stored into a memory or the like as a first speckle image. Next, a speckle field after displacement is captured, and the captured speckle field is stored into a memory or the like as a second speckle image. Then, a displacement of the target surface is measured based on results of comparison between the first speckle image and the second speckle image.
In the image correlation displacement sensor that measures a displacement by capturing speckle fields, a relatively high sample rate may be a desirable characteristic. The sample rate may depend on the size of the images acquired, the type of correlation calculations performed in order to determine displacement, and the amount of image data used in the correlation calculations. When linear displacement in only an X and/or a Y direction is desired, a relatively high sample rate may be maintained. However, when it is desired to measure and/or compensate for a rotational displacement (e.g., a rotation about a yaw axis which is normal to the X and Y axes, the correlation calculations may become time consuming and the sample rate may suffer. For example, “Submicron Deformation Field Measurements II: Improved Image Correlation,” G. Vendroux and W. G. Knauss, EXPERIMENTAL MECHANICS, Volume 38, Number 2, pp. 86-92 (hereinafter referred to as “Non-patent Document 1”) discloses that a displacement in X and Y directions and a rotation angle θ are measured. In Non-patent Document 1, however, it is concluded that relatively complicated rotational correlation calculations are needed in order to prevent significant errors in typical correlation measurements for determining displacement. However, this conclusion has adverse consequences in many applications, in that the associated calculation time increases, and the potential measurement sample rate is decreased. Such an outcome is undesirable in a number of applications, and particularly when displacement is more important than strain, and/or when relatively large or fast displacements are to be monitored.
It is an object of the present invention to provide an image correlation displacement sensor capable of measuring a rotation about a yaw axis with a simple method and at a high sample rate.